This invention relates to an improved method for forming a uniform coating of rubber or a synthetic resin on the external surface of a tubular textile jacket. More particularly, this invention relates to the use of an extending member capable of inflating the tubular textile jacket circularly in its cross-section when packed thereinto and keeping the tubular jacket almost circular in its cross-section during the method for forming a coating of rubber or a synthetic resin on the external surface of the tubular textile jacket.